total pressure and pressure drop

omaralyahia · July 14, 2015, 01:54

Hello every body
I have narrow Vertical Rectangular channel, heated from both side. the flow direction is downward flow, normal flow velocity (1 m/s) at the inlet B.C and average static pressure at the outlet, the total length of the flow channel is 0.3 m so the static pressure at the outlet will be ( rho*g*h=2935 Pa). The first problem is regarding to the pressure and pressure drop, whereas the total pressure at the outlet is lower than the total pressure at the inlet, but in reality at the flow velocity (1 m/s) the total pressure at the outlet should be the higher than the inlet due to the static pressure term (gravity effect). I make the flow in downward direction but I couldn't find how to specify the gravity direction except by actuating the buoyancy option On, but even if I used this option, the result not changing.

ghorrocks · July 14, 2015, 02:58

Don't forget that CFX uses pressure with the hydrostatic component removed when gravity is defined. See the documentation for more details on this. Look at Absolute Pressure if you want to see pressure with the hydrostatic component included.

Gravity is coupled to buoyancy. Why would you want to turn gravity on but not have buoyancy?

omaralyahia · July 14, 2015, 03:07

Thank you soo much
But how can I define the flow direction (Upward flow or downward flow) without the gravity. and how can I see the absolute pressure. In the CFX-post, I can found Pressure and Total Pressure as a variable

ghorrocks · July 14, 2015, 03:23

The flow direction is set by your boundary conditions and your simulation set up obviously. The whole point of CFD is that the solver tells you which direction the flow goes.

Look under the puzzlingly labelled "..." button to see the additional available variables.

omaralyahia · July 14, 2015, 03:28

Thaaaank you so much
I'm always looking in those additional variables but in fact it's my first time to find the absolute pressure. So could you please tell me what is the difference between the (Pressure, Total Pressure, and Absolut Pressure) in the CFX.
And regarding to the other question which I asked about the wall boiling model I already posted that

ghorrocks · July 14, 2015, 08:13

https://en.wikipedia.org/wiki/Pressure_measurement

July 14, 2015, 01:54	total pressure and pressure drop	#1
omaralyahia New Member Omar Sharief Join Date: Jun 2015 Posts: 7 Rep Power: 10	Hello every body I have narrow Vertical Rectangular channel, heated from both side. the flow direction is downward flow, normal flow velocity (1 m/s) at the inlet B.C and average static pressure at the outlet, the total length of the flow channel is 0.3 m so the static pressure at the outlet will be ( rhogh=2935 Pa). The first problem is regarding to the pressure and pressure drop, whereas the total pressure at the outlet is lower than the total pressure at the inlet, but in reality at the flow velocity (1 m/s) the total pressure at the outlet should be the higher than the inlet due to the static pressure term (gravity effect). I make the flow in downward direction but I couldn't find how to specify the gravity direction except by actuating the buoyancy option On, but even if I used this option, the result not changing.

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July 14, 2015, 02:58		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,712 Rep Power: 143	Don't forget that CFX uses pressure with the hydrostatic component removed when gravity is defined. See the documentation for more details on this. Look at Absolute Pressure if you want to see pressure with the hydrostatic component included. Gravity is coupled to buoyancy. Why would you want to turn gravity on but not have buoyancy?

July 14, 2015, 03:07		#3
omaralyahia New Member Omar Sharief Join Date: Jun 2015 Posts: 7 Rep Power: 10	Thank you soo much But how can I define the flow direction (Upward flow or downward flow) without the gravity. and how can I see the absolute pressure. In the CFX-post, I can found Pressure and Total Pressure as a variable

July 14, 2015, 03:23		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,712 Rep Power: 143	The flow direction is set by your boundary conditions and your simulation set up obviously. The whole point of CFD is that the solver tells you which direction the flow goes. Look under the puzzlingly labelled "..." button to see the additional available variables.

July 14, 2015, 03:28		#5
omaralyahia New Member Omar Sharief Join Date: Jun 2015 Posts: 7 Rep Power: 10	Thaaaank you so much I'm always looking in those additional variables but in fact it's my first time to find the absolute pressure. So could you please tell me what is the difference between the (Pressure, Total Pressure, and Absolut Pressure) in the CFX. And regarding to the other question which I asked about the wall boiling model I already posted that

July 14, 2015, 08:13		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,712 Rep Power: 143	https://en.wikipedia.org/wiki/Pressure_measurement